Most small commercial vehicles and particularly automotive and mobile air conditioning systems utilize a fixed area refrigerant expansion device for flow control to reduce or eliminate moving parts and reduce costs. Known fixed area expansion devices are long capillary tubes or relatively short orifice tubes in the high-pressure liquid line between the condenser and evaporator. The refrigerant flow is primarily dependent on the inlet pressure (head pressure) at the orifice and the amount of liquid subcooling. In fixed area expansion devices, increasing head pressure or subcooling increases flow while increasing gas at the inlet decreases flow. Suction pressure changes have no effect on flow in normal system operation at higher ambient since flow is at "sonic" velocity and suction pressure is usually below the pressure at which this sonic flow occurs. As described in detail in my U.S. Pat. No. 5,901,750, incorporated herein by reference, these characteristics make the fixed area expansion device or capillary tube self regulating and produces adequate performance in a wide range of conditions encountered in normal automotive air conditioner operation with the exception performance at idle. In automotive use the fixed orifice tube is sized for high ambient load at high speed vehicle operation and is generally about twice as large at idle as is optimal, resulting in increased compressor horsepower and a reduction in cooling.
Another known expansion device for use flow control in refrigerant systems is a thermostatic expansion valve. A thermostatic expansion valve is a variable area expansion device and operates to control the flow rate of liquid refrigerant entering the evaporator as a function of the temperature of the refrigerant gas leaving the compressor. However, while fixed area devices cannot match the efficiency of the thermostatic expansion except at certain defined operating conditions, thermostatic expansion valves are more complicated to manufacture and thus, more costly than the fixed devices.
An example of an automotive refrigerant system operation at high vehicle operation speed as compared to vehicle idle is as follows: At high operation speeds the head pressure may be approximately 250 PSIG and the sub-cooling temperature 20.degree. F. with a system capacity of approximately 24,000 BTU/Hr. At idle the head pressure of the vehicle rises to 350 PSIG due to reduction of condenser air flow and re-circulation of hot under-hood air into the condenser. Accordingly, balance is achieved at idle by reducing sub-cooling at the expansion device inlet. Initially, the rise in head pressure increases flow through the orifice tube. The result is that the sub-cooled liquid in the condenser is flushed out and uncondensed gas enters the orifice tube inlet. Gas greatly reduces flow until the flow rate out of the compressor is equal to flow through the orifice tube. Since capacity per pound of refrigerant is a function of liquid percentage after expansion, a gaseous mixture entering the expansion device greatly reduces capacity and system efficiency. At idle this capacity reduces to 12,000 BTU/Hr. in this example. If a smaller flow area is utilized in the expansion device refrigerant flow is reduced allowing liquid backup in the condenser and thus sub-cooling to occur. The net result is more cooling at reduced compressor work.
However, when small orifice are used at high load high speed operational conditions, more refrigerant is required in the system as more back up of liquid occurs until enough subcooling and head pressure are available to again flow enough to satisfy evaporator requirements. Unfortunately, this causes the head pressure to rise in the range of 300-400 PSIG, substantially reducing the compressor life. Accordingly, the variable flow orifice must not be engaged into the minimum flow area at high speed high load conditions, in order to operate satisfactory.
Further, a very short orifice tube relative to it's diameter becomes more restrictive as sub-cooling is increased. This characteristic of the short orifice tube is a detriment in automotive use since flow starvation may occur at high vehicle operational speeds if the small orifice size is engaged. Starvation results in high compressor discharge superheat temperatures, which may be very detrimental to compressor durability.